The present invention relates to the improvement of a speed shift control system of a continuous transmission.
As a conventional art for changing the gear ratio of a continuous transmission, for example, the Japanese Patent Publication Gazette No. 63-42146 discloses the known device having a pair of, primary and secondary, pulleys, effective diameter of both pulleys are changable, and a belt coupling the above two pulleys. In this device, each effective diameter of the primary and the secondary pulley is continuously regulated by providing/releasing a hydraulic pressure to/from a hydraulic cylinder of the primary pulley so as to control the gear ratio changeable.
Another art of controlling the gear ratio to a target gear ratio is a feedback-control which controls the primary pulley speed to a target speed. In other words, the target primary pulley speed which corresponds to the target gear ratio is determined first according to the driving status, and the actual primary pulley speed is detected. On the basis of the deviation between the target and the actual primary pulley speeds, a feedback-control value is gained. The effective diameter of the primary pulley is controlled by the feedback-control value so as to make the primary pulley speed to the target primary pulley speed. Thus, the gear ratio is controlled to the target gear ratio.
However, the following disadvantages were found in gear ratio control device for a continuous transmission by the conventional art. As shown in FIG. 10, in the gear ratio map of which X axis shows the secondary pulley speed and the vehicle speed and Y axis shows the primary pulley speed, when the gear ratio is, for example, increased from 0.5 to 0.7, at point A in the map, i.e. the secondary pulley speed is 5000 r.p.m and the primary pulley speed is 2500 r.p.m, the deviation between the actual and the target (3500 r.p.m) primary pulley speeds is 1000 r.p.m at 0.7 gear ratio. On the other hand, when the gear ratio is increased from 0.5 to 0.7 as the above example at point B, i.e. the secondary pulley speed is 3000 r.p.m and the primary pulley speed is 1500 r.p.m, the deviation between the actual and the target (2100 r.p.m) primary pulley speed is 600 r.p.m at 0.7 gear ratio, in spite of the gear ratio is increased by 0.2 as the above example. This means that the deviation between the actual and the target speed varies according to the primary pulley speed and the secondary pulley speed even the gear ratio is changed by the same amount. The above example can be applied to the case having changed the gear ratio by the same amount at different gear ratio. For example, when the gear ratio is changed from 1.8 to 2.0, the gear ratio is changed by the same amount, 0.2, as the above example, from 0.5 to 0.7. However, when gear ratio is changed at point C having the same primary pulley speed (2500 r.p.m) as the point A and the secondary pulley speed is about 1400 r.p.m, the deviation between the actual and the target (2800 r.p.m) primary pulley speed is 300 r.p.m which is greatly different from the deviation 1000 r.p.m at point A. Thus, when gear ratio is changed, the deviation varies depending on the primary pulley speed and the secondary pulley speed. Since the feedback-control value is determined in such a way that the speed deviation is multiplied by the control gain, the greater deviation results in the greater feedback-control value and consequently, speed for changing the gear ratio will be faster even the gear ratio is changed by the same amount. On the other hand, the lesser speed deviation results in the lesser feedback-control value and consequently, speed for changing the gear ratio will be slower. Thus, different control gains are obtained at each gear ratio change and the speed differs at each gear ratio change. As a result, different speed response is gained at each gear change and accordingly, a problem will be caused in driving a vehicle.
The object of the present invention is to maintain the constant speed for changing the gear ratio at all times in spite of the primary pulley speed and the secondary pulley speed.
To achieve the above object, when the same amount of gear ratio is changed, the feedback-control value is kept equal by changing the control gain according to the speed deviation which varies depending on the primary and the secondary pulley speeds.
Concretely, according to the present invention, the continuous transmission comprises a pair of, first and secondary, pulleys having the changeable effective diameters, a belt coupling both pulleys, and regulator means for regulating the effective diameters of the primary and the secondary pulleys. Also, it comprises target speed determining means for determining a target primary pulley speed corresponding to a target gear ratio varying depending on the driving status, speed detecting means for detecting an actual primary pulley speed, and control means which receive outputs from the target speed determining means and the speed detecting means and feedback-controls the regulator means for controlling the primary pulley speed to the target primary pulley speed on the basis of the deviation between the actual and the target primary pulley speeds. Moreover, it comprises control gain changing means which changes the control gain of the above control means according to the size of the deviation of the actual and the target primary pulley speeds when changing the gear ratio.
According to the above structure, in the present invention, when the gear ratio is changed, for example, from 0.5 to 0.7 at point A in FIG. 10, a control gain is controlled to be small by the control gain changing means, while the speed deviation is large. On the other hand, when the gear is changed at point B, the control gain is changed to get larger since the deviation of the pulley speed is smaller than that of point A. As a result, since the feedback-control value obtained by multiplying the deviation of the pulley speed to the control gain are equal at points A and B in FIG. 10, a substantial control gain is equal even the deviation of pulley speed is different from each other and accordingly, a constant speed for changing gear ratio can be maintained by the equal substantial control gain, even the speed deviation is different.
Moreover, in case of the large speed deviation, since the feedback-control value is not too large but suitable, the hunting of the primary pulley speed occurred around the target speed can be prevented.
In the above description, although the control gain is changed directly according to the speed deviation, the control gain can be changed only by the primary pulley speed, or only by the secondary pulley speed, or by the both pulleys speed since the speed deviation changes according to the actual and the target primary pulley speed and the actual secondary pulley speed.
The above object and features of the present invention will be more apparent with the description of the prefered embodiment with reference to the accompanying drawings.